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August 29, 2008
Source:
American Society of Plant
Biologists
Demand for corn -- the world's
number one feed grain and a staple food for many -- is
outstripping supply, resulting in large price increases that are
forecast to continue over the next several years. If corn's
intolerance of low temperatures could be overcome, then the
length of the growing season, and yield, could be increased at
present sites of cultivation and its range extended into colder
regions.
Drs. Dafu Wang, Archie Portis, Steve Moose, and Steve Long in
the Department of Crop Sciences and the Institute of Genomic
Biology at the University of
Illinois may have made a breakthrough on this front, as
reported in the September issue of the journal Plant Physiology.
Plants can be divided into two groups based on their strategy
for harvesting light energy: C4 and C3. The C4 groups include
many of the most agriculturally productive plants known, such as
corn, sorghum, and sugar cane. All other major crops, including
wheat and rice, are C3. C4 plants differ from C3 by the addition
of four extra chemical steps, making these plants more efficient
in converting sunlight energy into plant matter.
Until recently, the higher productivity achieved by C4 species
was thought to be possible only in warm environments. So while
wheat, a C3 plant, may be grown into northern Sweden and
Alberta, the C4 grain corn cannot. Even within the Corn Belt and
despite record yields, corn cannot be planted much before early
May and as such is unable to utilize the high sunlight of
spring.
Recently a wild C4 grass related to corn, Miscanthus x
giganteus, has been found to be exceptionally productive in cold
climates. The Illinois researchers set about trying to discover
the basis of this difference, focusing on the four extra
chemical reactions that separate C4 from C3 plants.
Each of these reactions is catalyzed by a protein or enzyme. The
enzyme for one of these steps, Pyruvate Phosphate Dikinase, or
PPDK for short, is made up of two parts. At low temperature
these parts have been observed to fall apart, differing from the
other three C4 specific enzymes. The researchers examined the
DNA sequence of the gene coding for this enzyme in both plants,
but could find no difference, nor could they see any difference
in the behavior of the enzyme in the test tube. However, they
noticed that when leaves of corn were placed in the cold, PPDK
slowly disappeared in parallel with the decline in the ability
of the leaf to take up carbon dioxide in photosynthesis. When
Miscanthus leaves were placed in the cold, they made more PPDK
and as they did so, the leaf became able to maintain
photosynthesis in the cold conditions. Why?
The researchers cloned the gene for PPDK from both corn and
Miscanthus into a bacterium, enabling the isolation of large
quantities of this enzyme. The researchers discovered that as
the enzyme was concentrated, it became resistant to the cold,
thus the difference between the two plants was not the structure
of the protein components but rather the amount of protein
present.
The findings suggest that modifying corn to synthesize more PPDK
during cold weather could allow corn, like Miscanthus, to be
cultivated in colder climates and be productive for more months
of the year in its current locations. The same approach might
even be used with sugar cane, which may be crossed with
Miscanthus, making improvement of cold-tolerance by breeding a
possibility.
This research was supported by a grant from the National Science
Foundation. |
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